Electron discharge device



M. J. KELLY ELECTRON DISCHARGE DEVICE Sig Filed Dec. 31, 1926 Ewfii lliili 2.

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June 14, 1932.

Patented June 14, 1932 UNITED STATES PATENT OFF! MERVIN J. KELLY, OF- NEW YORK, N. Y., ASSIGNOB TO WESTERN ELECTRIC COMPANY, INGOBPORATED, OF NEW YORK, 11'. Y., A CORPORATION OF NEW YORK ELECTRON DISCHARGE DEVICE Application filed December 31, 1926. Serial No. 158,167.

This invention relates to thermionic electron discharge devices and methods of manufacture.

The invention provides an electron discharge device of the coated filament type having a long life and uniform filament emission, and a process by which such device may be obtained.

The invention relates to vacuum tubes of i the coated filament type in which the anode and grid are made of nickel or other metal containing no carbon near their surfaces. A method of freein the grid and anode surface layers of car on consists in heating the grid and anode in the presence of oxygen to form a coating of black oxide of nickel on the electrodes. The oxidized electrodes are then heated in a reducing atmosphere to clean the nickel. The filament coating is a mixture of barium andstrontium oxides or other material of similar characteristics.

The elimination of carbon prevents the formation of carbon oxides during the evacuating process and permits the bombardment time required to obtain a tube as hard as the previous standard tubes to be reduced from two hours to approximately ten minutes; but the thermionic activity is also reduced.

In accordance with the present invention such a tube as justdescribed in which the surface layers of the anode and grid are freed of carbon is greatly improved by vaporizing therein an electropositive metal, preferably magnesium, during the exacuating process. This results in restoring the thermionic activity without sacrificing the advantage gained from eliminating the carbon oxides. Moreover, the emission from the fila- 40 ment is caused to remain substantially constant over a long period instead of slowly decreasing an heretofore, and the filament is so much more uniform that it is possible to desi a tube for the same service with from onealf to one-quarter the amount of filament previously used. The vaporizing of the electropositive metal is preferably carried out during the final stages of the evacuating process.

A more detailed description of the invention follows and is illustrated in the accompanying drawing.

Fig. 1 illustrates an electron discharge device embodying the structural features of this invention.

Fig. 2 is a graphic representation for comparison of the life characteristic of a particular type of discharge device with that of a similar type of discharge device made in accordance with this invention.

This invention will be better understoo from the following brief statement of the usual procedure employed in evacuating such devices, the characteristics of the ordinary vacuum tube employing a coated filament and a grid and anode made from commercial nickel from which carbon has not been eliminated, and the effect of eliminating carbon from the surface layers of the grid and anode.

It is usual to employ in devices of this type a filament of platinum alloy coated with barium and strontium oxides. Heretofore the anode and grid were made of the best grade of commercial nickel, which contains from 0.05 to 0.025% carbon. This carbon is introduced into the nickel in its metallurgical treatments and all commercial nickel contains it. In the process of evacuating tubes they are sealed to a high vacuum exhaust station. The glass vessel is baked out at a temperature of about 400 C. while the pumps are in operation. The filaments are outgassed by heatlng them with a current considerably greater than the operating current. The grid and plate electrodes are denuded of gas by bombarding them with electrons from the filament. After the bombardment is completed the tubes are sealed ofif from the high vacuum exhaust station. The clean-up of the grid and plate by bombardment when made of commercial nickel requlres approximately two hours.

The thermionic activity of the filament in tubes made according to this practice varied over more than a tenfold range. In the design of commercial tubes using this filament,

enough filament had to be used to provide the required electron emission from the lowest quality filament. This meant that there was several times the required emission from the filament of the upper range of quality. The

value that they life characteristic of this type of filament is shown in curve A of Fig. 2. It should be noted that when the average life of the tubes was reached only about 38% of them were left in service. The tubes were retired from service when the emission decreased to such a could not perform their function.

The decarbonization of commercial nickel electrodes is accomplished by first heating the electrodes in an open oven or in the presence of oxygen for approximately one half hour at a temperature of the order of 950. This heat treatment results in the formation of a coating of black oxide of nickel on the surface of the electrodes and oxides of carbon to design a which pass ofi as gases. The oxidized electrodes are then again heated to a; temperature of the order of 900 in a reducing atmosphere. This operation may be earned on in a. vacuum oven, the evacuation of .which is accomplished by an oil pump or in a vessel through which hydrogen is passed. In the first instance, sufiicient gaseous hydrocarbon is present in the vessel from the oil pump to reduce the oxide of nickel, whereas in the second case the hydrogen acts as a reducing agent. This operation cleans the nickel oxide from the surfaces of the electrodes so that they are in proper condition to be mounted in an electron discharge device.

By the method of oxidation and subsequent reduction of nickel to free a surface layer of carbon heretofore described, the bombardment time required to obtain a tube as hard as the previous standard tube was reduced from two hours to approximately .ten minutes. It was found, however, that the possible, how this decrease in thermionic activity could be prevented. It Was discovered that not onlycould the thermionic activity be restored without sacrificing the advantage gained from eliminating the carbon oxides but the emission from the filament could be caused to remain substantially constant over a long period instead of slowly decreasing as heretofore, and a. filament could be produced which is so uniform that it is possible tube for the same service with one-half to one-quarter the amount-of fila-i ment previously used. This result is accomphshed by vaporizing an electropositive metal such as magnesium or aluminum in vacuum during the ten minute bombardment interval and preferably at or near the completlon of that interval.

The thermionic activity of the best filament made in accordance with this invention is approxlmately the same as that of the best filament activated by the previous process, but the poorest filament has about one-fourth the activity of the best, where the poorestfilament of the previous method had an activity of about one-tenth of the best. Due to this narrowing of the range of filament activity keeping the upper level of activity the same, it is possible, as stated above, to design a tube for the same'service with one-half to onequa'ter the amount. of filament previously use The life behavior of filaments in tubes with carbon free elements and activated by the vaporized magnesium is entirely different from that of the filaments in tubes made accordingto the previously followed process. The activity of the filaments of the former tubes decreased with giving a life curve of the type hereinbefore referred to, like curve A in Fig. 2. The act1v1ty of the filaments in the tubes having carbon-free elements and vaporized magnesium either increases with time or remains constant. When the activity begins to decrease, the reduction is very much more rapid than in the former tubes. The life of such tubes is therefore very much increased and the earlier failures due toloss of filament act ivity is almost completely eliminated. The hfecurve of tubes made according to this invention is well represented in curve B of Fig. 2. urve B is an estimated curve drawn for comparison with curve A and is based partially on normal tests of tubes of the same type as those tested for curve A and partially on accelerated life tests.

The average life of the tubes represented by curves such as A and B of Fig. 2 is the area under the curve. For example, the area under the curve A is the average life of the tubes represented by curve A. This is also the area of the rectangle, bounded by the coordinates, the horizontal dotted line through unity (100%) and the average life line of curve A. Hence it follows that the area to the left of the average life line and between the curve A and the dotted line through 100% equals the area to the right of the average life line below the curve A.

Magnesium vapor has heretofore sonictimes been used to assist in cleaning up gases during the process of evacuation but not in connection with tubes that employ coated filaments,

time at varying rates so far as known, and not'for the aseaaea purpose of activating the thermionically active material.

One type of vacuum tube to which this invention is applicable is shown in Fig. 1. It comprises an enclosing vessel 10 having a reentrant stem 11 terminating in a press 12 in which are sealed the leading-in wires for the electrodes. A glass arbor or post 13 is fused to the side of the stem and extends substantially parallel to the axis thereof. A filament 14 is resiliently supported from the arbor 13 and preferably consists of a platinum nickel core such as is disclosed in J. E. Harris Patent No. 1,542,385, issued June 16, 1925 and coated with thermionic active material in ac-- cordance with the process disclosed in C. D. Hooker Patent No. 1,545,256, issued July 7, 1925. A. nickel grid electrode 15 and a nickel anode or plate electrode 16 hav1ng surfaces free from carbon are supported by wires extending from the arbor 13 in cooperative relation to the filament. A magnesium ribbon 17 is entwined on a metal ring 18 which is welded to the plate electrode 16. After the assembly of the electrodes in the vessel 10, the device may be connected to a mechanical pumping system by means of the tubulation 19 and the gas removed from the interior of the vessel in the manner hereinbefore described. As a final step in the exhaust process prior to the sealing off of the tubulat on, the magnesium ribbon 17 is vaporized by high frequency induction heating in the manner disclosed in M. J. Kelly Patent No. 1,565,857, issued December 15, 1925.

The invention is applicable to other types of vacuum tubes than that shown in Fig. 1 and it is to be limited only by the scope of the appended claims.

What is claimed is:

1. An electron discharge device comprising a cathode coated with thermionically active material, a carbon-free electrode cooperating with said cathode, and an electropositive substance within said device capable of activating said coated cathode whereby the emissionfrom said cathode does not decrease appreciably over a long period and then decreases rapidly.

2. An electron discharge device comprising a cathode coated with thermionically active material, a carbon-free anode in operative relation to said cathode, and a vaporized activating substance acting on said cathode whereby the emission from said cathode does not decrease appreciably over a long period, said cathode having been activated In an atmosphere substantially free from oxides of carbon.

3. An electron discharge device comprising .a cathode coated with thermionically active alkaline earth oxide, a grid and an anode associated with said cathode, said grid and anode being entirely free from surface carbon, and vaporized magnesium in said device whereby the emission from said cathode does not decrease appreciably over a period of at least several thousand hours and then decreases rapidly.

4. An electron discharge device comprising a cathode coated with thermionically active barium and strontium oxides, the emission from which does not decrease appreciably over-a period of at least several thousand; hours and then decreases rapidly, said cathode having been activated in an electropositive metal vapor atmosphere substantially free from the oxides of carbon, and a decarbonized anode spaced in operative relation to said cathode.

5. In an electron discharge device, a cathode comprising a metallic element coated with thermionically active barium and strontium oxides, the emission from which does not decrease appreciably over a period of at least several thousand hours and then decreases rapidly, said cathode having been activated in a vapor atmosphere of magnesium substantially free from the oxides of carbon, and a carbon free nickel anode.

6. The process of making an electron discharge device comprising an enclosing vessel containing a cathode coated with thermionically active material and another electrode, which method comprises removin surface carbon from said other electr e associated with said cathode, and activating said material Within said vessel in an atmosphere of vaporized electro-positive metal at low va or pressure.

The process of making an electron discharge device comprising an enclosing vessel containing a cathode coated with thermionically active material and an anode, which method comprises decarbonizing said anode, and activating said material within said vessel in an atmosphere of vaporized electropositive metal at low vapor pressure free from carbon'oxides.

8. The process of making an electron discharge device comprising an enclosing vessel containing a cathode coated with thermionically active material and an anode, which method comprises oxidizing said anode, reducing said oxide to free said anode from surface carbon, introducing magnesium in said vessel, and vaporizing said magnesium.

9. The process of making an electron discharge device comprising an enclosing vessel containing a cathode coated with thermionically active material and an anode, which method comprises coating said anode in an oxidizing atmosphere, removing said coating ina reducing atmosphere to provide a clean surface on said anode free from carbon, introducing an electro-positive metal having a low vapor pressure in said vessel, evacuating said vessel, and vaporizing said metal during the period of evacuating.

10. The process of making an electron discharge device comprising an enclosing vessel containing a cathode coated with barium and strontium oxides, and a grid and anode having metallic surface layers free from car- 5 bon, which method comprises heating said grid and anode in an oxidizing atmosphere to approximately 950, heating said grid and anode in a reducin atmosphere at approximately 900, intro ucing magnesium in said vessel, evacuating said vessel, and vaporizing said magnesium during the last portion of the evacuating period.

In Witness whereof, I hereunto subscribe my name this 29th day of December, A. D. 926.

IVERVIN J. KELLY. 

